Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on a print medium in a particular order to form alphanumeric characters, area-fills, and other patterns thereon. Low cost and high quality of the hardcopy output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers.
The non-impact printing process of ink-jet printing involves the ejection of fine droplets of ink onto a print medium such as paper, transparency film, or textiles in response to electrical signals generated by a microprocessor. There are two basic means currently available for achieving ink droplet ejection in ink-jet printing: thermally and piezoelectrically. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor.
In thermal ink-jet printing, an ink-jet image is formed when a precise pattern of dots is ejected from a drop generating device known as a "printhead" onto a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles (or ejector portions) attached to a thermal ink-jet printhead substrate, such as silicon, nickel, or polyimide, or a combination thereof. The substrate incorporates an array of firing chambers or drop ejector portions that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoir. Each firing chamber has a thin-film resistor, known as a "firing resistor," located opposite the nozzle so ink can collect between the firing resistor and the nozzle. The printhead is mounted on a carriage that travels along the width of the printer (otherwise referred to as the "scan axis").
Commercially-available thermal ink-jet printers, such as DeskJet.RTM. printers available from Hewlett-Packard Company, use inks of differing hues, namely, magenta, yellow, and cyan, and optionally black. The particular set of colorants, e.g., dyes, used to make the inks is called a "primary dye set." A spectrum of colors, e.g., secondary colors, can be generated using different combinations of the primary dye set.
One category of ink-jet printers utilizes disposable printheads in which the ink reservoirs are on-board the carriage, thus the term on-board or on-axis. The reservoirs can be formed integrally with the printhead portions or they can be detachably connected thereto.
Another category of ink-jet printers employs ink reservoirs that are not located on the carriage, thus the term off-board or off-axis. In one case, the reservoir intermittently replenishes the printhead with ink when the printhead travels to a stationary reservoir periodically for replenishment. Another type makes use of a replaceable ink reservoir connected to the printhead by a fluid conduit. The printhead is replenished with ink through this fluid conduit.
The reservoirs may be individually (separate from other reservoirs) replaceable or the reservoirs can be formed as one integral reservoir portion, to be replaced as a unit.
Different printhead/ink reservoir configurations address different customer needs. For example, on-board designs provide for ease of use. Printers using the off-board designs provide for less interruptions during printing jobs which require larger ink volumes, such as large format printing.
In general, a successful ink set for color ink-jet printing should have the following properties: good crusting resistance, good stability, the proper viscosity, the proper surface tension, good color-to-color bleed alleviation, rapid dry time, no negative reaction with the vehicle, consumer-safety, and low strike-through. When placed into a thermal ink-jet system, the ink set should also be kogation-resistant.
Regardless of whether an ink is dye-based or pigment-based, ink-jet inks commonly face the challenge of color-to-color or black-to-color bleed control. The term "bleed," as used herein, is defined to be the invasion of one color into another, once the ink is deposited on the print medium, as evidenced by a ragged border therebetween. Bleed occurs as colors mix both on the surface of the paper substrate as well as within the substrate itself. The occurrence of bleed is especially problematic between a black ink and an adjacently-printed color ink because it is all the more visible. Hence, to achieve good print quality, bleed should be substantially reduced or eliminated such that borders between colors are clean and free from the invasion of one color into the other. Several approaches have been utilized in controlling bleed between the printed images, many of which utilize reactive ink mechanisms.
One approach used for controlling bleed between the printed images, as disclosed in U.S. Pat. No. 5,428,383, entitled "Method and Apparatus for Preventing Color Bleed in a Multi-Ink Printing System," filed by Shields et al., and assigned to the same assignee as the present invention, and incorporated herein by reference, is to employ a precipitating agent (e.g., a multi-valent metal salt) in one ink, and a colorant, preferably in the form of an organic dye having at least one and preferably two or more carboxyl and/or carboxylate groups, in another ink, preferably the black ink. When the inks are printed on the printing medium adjacent one another, the ink containing the precipitating agent brings about the precipitation of the colorant with the carboxyl/carboxylate group, thereby preventing the migration of the colorant in the other ink, thereby reducing bleed between the two adjacently printed areas.
Another method of reducing bleed between ink-jet inks involves the use of pH-sensitive dyes as disclosed in U.S. Pat. No. 5,181,045 entitled "Bleed Alleviation Using pH-sensitive Dyes/Inks," filed by Shields et al, and assigned to the same assignee as the present invention, and incorporated herein by reference. It disclosed therein that an ink having a pH-sensitive dye, the "pH-sensitive ink," would be prevented from bleeding into an adjacent ink having an appropriate pH, the "target ink." More particularly, migration of the ink having the pH-sensitive dye is prevented by rendering the dye insoluble on the page by contact with the adjacent ink having the appropriate pH. Thus, bleed is reduced or eliminated by using both the "pH-sensitive" ink as well as the "target" ink. Typically, since the invasion of a black dye into a color ink is more problematic than vice versa because of its greater visibility, the black ink would employ the pH-sensitive dye and the pH of the color ink would be controlled in the practice of the invention, such that the black ink would be prevented from bleeding into the color ink. The method of U.S. Pat. No. 5,181,045 requires a pH differential of about 4 (or even 5) units to completely control bleed.
U.S. application Ser. No. 08/567974, entitled "Bleed Alleviation in Ink-Jet Inks using Organic Acids," filed by Adamic et al. on Dec. 6, 1995, and assigned to the same assignee as the present invention) and U.S. Pat. No. 5,679,143 (entitled "Bleed Alleviation in Ink-jet Inks Using Acids Containing a Basic Functional Group," filed by Looman and assigned to the same assignee as the present invention), both incorporated herein by reference, further disclose methods for controlling bleed by forcing the precipitation of a pH-sensitive dye in one ink (the pH-sensitive ink) on the print medium by contacting the pH-sensitive dye with a second ink (the target ink) having an appropriate pH (either higher or lower than the first ink). Upon contact on the print medium, the pH-sensitive dye of the first ink becomes insoluble, thus bleeding less. U.S. application Ser. No. 08/567974 discloses the use of organic acids to reduce the pH differential required to effect precipitation of a pH-sensitive dye colorant as compared to that disclosed in U.S. Pat. No. 5,181,045, described above. U.S. Pat. No. 5,679,143 employs the use of an organic acid having no basic functional groups and a pH adjusting organic compound containing both acidic and basic functional groups, specifically, at least one acidic functional group and at least one basic functional group, where the number of basic functional groups is the same as or greater than the number of acidic functional groups. The presence of the organic acid in the ink-jet ink composition reduces the pH differential required to render insoluble the pH-sensitive colorant of a second encroaching ink-jet ink composition, as described in U.S. application Ser. No. 08/567974 above. The presence of the dual-function pH adjusting compound further increases the concentration of an acid functional group in the ink-jet ink composition while also increasing the pH of the ink-jet ink composition to acceptable levels. Therefore, the dual-function pH adjusting compound augments the bleed alleviation achieved by an organic acid alone according to the mechanism disclosed in Ser. No. 08/567,974.
In order to prevent the occurrence of bleed between the primary (e.g., cyan, magenta, and yellow) and secondary colors (e.g., red, blue, and green) with black, all the three primary colors are designed to be reactive with the black ink according to reaction mechanisms such as those described above (multi-valent metal salt, pH-sensitive dye).
The above solutions utilizing reactive inks, although of merit, do not maximize flexibility in the design of the inks and printing system in which they are used. For example, using reactive ink systems may lead to unwanted mixing of the reactive inks, hence contributing to reliability problems in the printing system, particularly in compact printhead arrangements or compact ink delivery arrangements.
Therefore, a need exists for an ink-jet printing system and ink supply configuration that can utilize the advantages of reactive inks while allowing for maximum flexibility in the design and architecture of the ink-jet printing system.